I second what the Boss said. Basically, there is not a lot of research out there on exercise during pregnancy, for obvious reasons. It is not a comfortable (or ethical) state to study. I was very interested in this topic and did several presentations on it during residency training. The most recent ACOG (American College of Obstetrics & Gynecology) guideline is that
in normal/non-complicated pregnancies (make sure that your doc validates that this is you), it is advised to moderately exercise (to keep your heart rate below 85% of your max heart rate which, is approximately 220-your age) up to 30 minutes per day on most days of the week.
ACOG recommends that after the first trimester, women should avoid full contact sports (soccer, tackle football, snowboarding, extreme cycling, etc). Most activities are safe, but those that pose a risk of abdominal trauma should be avoided. Similarly, scuba diving should be avoided not because of risk for injury, but because the baby's immature circulatory system increases the baby's susceptibility to decompression sickness. Despite older traditional recommendations, running is ok but again, try to stay below 85% of your max heart rate.
It is also recommended to steer clear of weightlifting greater than about 15 pounds. The reason for this is that when lifting heavy weights, your body tends to forcibly exhale against a closed glottis, which increases intrathoracic pressure and decreases venous return to the heart. Under nonpregnant physiological circumstances, this is ok but when pregnant, this could compromise oxygen delivery to your baby's developing organs.
Although there is inadequate research on strenuous exercise, athletes who continue to train at a moderate level during an uncomplicated pregnancy are considered safe. Basically, if you are an athlete or a generally active person, you can continue to exercise but at slightly lower intensity during pregnancy. If you are not an athlete and do not exercise regularly prior to pregnancy, you are encouraged to begin an exercise regimen. This will absolutely make both you and your baby healthy. You should be reasonable about the new regimen. As in, start at 15 minutes three days per week and work up from there, not to exceed 85% of your maximum heart rate. Before beginning an exercise regimen (as always, even when not pregnant), you should check with your doctor to make sure that you are otherwise healthy (i.e. that you do not have an abnormal cardiovascular or respiratory disease).
Non-strenuous exercise during the postpartum period has been shown to reduce postpartum depression. Because it is known that weight loss at a moderate pace does not reduce the milk supply, it is safe to resume exercise during the months of breastfeeding.
